LONDON — Michael Hunter was trying to deliver on a promise he made to his dying father.

Dominic Breazeale was trying to complete the remarkable transformation from college football quarterback to medaling as an Olympic boxer.

Both U.S. Olympic boxers had gold on their minds and, in falling short with opening-bout losses yesterday at the ExCel Center in East London, both provided a sobering snapshot of how far away U.S. boxing is from being a world power — particularly in the heavyweight division, where America’s Olympic glory days of Cassius Clay before he became Ali, George Foreman and the Spinks brothers seem like lifetimes ago.

Both Hunter, the son of the late Michael “The Bounty’’ Hunter, a former heavyweight champion, and Breazeale, who played quarterback at the University of Northern Colorado, failing in his attempt to make it in the NFL, lost to Russian boxers with more skill and experience.

After Hunter, fighting as a heavyweight, looked tired early in his loss to Artur Beterbiev, Breazeale looked raw and unpolished and was often out of balance in the ring in a super heavyweight fight and was completely outclassed by Magomed Omarov.

For Breazeale, who had a 10-day tryout with the Giants after he got out of college in 2008, as exhilarating as those three rounds were, he said they were tougher than anything he’s ever done in football.

“It’s night and day,’’ he said. “With the game of football, you play it. In boxing, you’ve got to live it. When you’re in the ring there’s no blaming anyone else. You either get the win and all the glory or you get the loss and all the blame.’’

Breazeale conceded that he misses football, which he began playing when he was a six year old.

“To give it up and close the door on it … it definitely took me some time to harp and dwell on it and I had crying nights,’’ he said. “I definitely miss it. But the door just closed. The NFL came knocking. I got my opportunity and things didn’t pan out the way I wanted them to.’’

So the 6-7, 255-pound Breazeale, who started for Northern Colorado in 2006 and 2007, turned to boxing after getting a random recruiting call from All American Heavyweights, a program created to widen the talent pool of U.S. amateur boxing by attracting athletes in other sports.

“Boxing was the next best thing,’’ he said. “And in 3 1⁄2 years I got to the Olympics.’’

Breazeale and Hunter both said yesterday it is their intention to turn pro, meaning they won’t be around in four years as better-developed boxers to help Team USA, and therein lies a big problem with the program. Boxers get into the game more for the money than the medals.

“It’s very disappointing, because I always had a promise with my father before he passed that I would win a gold medal and I happened to fail that mission,’’ Hunter, who tried and failed to make the 2008 Olympic team, said. “I’m definitely going to turn professional. I’m kind of done with [the] amateur game and ready to move on to [the] next chapter of my life.’’

So there go two more promising U.S. heavyweight boxing hopes in Hunter and Breazeale.

“This kid has come from taking snaps in football to the biggest stage in amateur boxing,’’ Charles Leberette, one of the U.S. boxing coaches, said of Breazeale. “That says a lot about his athletic ability and his will to persevere. With his personality and his work ethic, once his skills catch up with his will to perform he’s going to be a great boxer.’’